1. Field of the Invention
The invention relates to an operation control map, a display unit, and a white line detection apparatus. More particularly, the invention relates to an operation control map, a display unit, and a white line detection apparatus that enable a white line detection portion to perform sufficiently within a range where the effect from thermal noise is acceptable.
2. Description of the Related Art
In recent years, white line detection apparatuses have been developed that detect white lines on roads based on captured images of the road surface. Some white line detection apparatuses according to related art include i) an imaging portion that includes a solid-state image sensor for capturing an image of the road surface, ii) a gain controlling portion that controls the gain when amplifying a road surface image signal generated by the imaging portion, based on the brightness around the vehicle, iii) a white line detection portion that compares the difference between the brightness values of the road and the white line in the amplified road surface image signal with a threshold value and detects the white line based on the result of the comparison, iv) a temperature measuring portion that measures the temperature of the imaging portion, v) a determining portion that determines whether to allow operation of the white line detection portion, and vi) an operation controlling portion that controls the operation of the white line detection portion based on the determination results of the determining portion.
The reason why the determining portion determines whether to allow operation of the white detection portion based on the temperature of the imaging portion is as follows. That is, thermal electrons are produced in a light receiving portion (i.e., a photodiode) of the solid-state image sensor. The quantity of thermal electrons gradually increases as the temperature of the imaging portion rises. Accordingly, if the temperature of the solid-state image sensor is high, a large quantity of thermal electrons accumulates in the light-receiving portion. If the voltage based on the thermal electrons is amplified by the amplifying portion, a large amount of thermal noise is produced. As a result, the white line detection portion becomes unable to distinguish the white line from the thermal noise so the identifying accuracy of the white line diminishes. Therefore, if the temperature of the image portion exceeds a predetermined temperature, the determining portion according to the related art will not allow the white line detection portion to operate at all.
In order to facilitate detection of the white line, the gain of the amplifying portion is controlled to gradually increase as the brightness around the vehicle decreases. Because thermal noise is amplified by the amplifying portion, the effect of the interference from thermal noise on the accuracy of the white line detection increases as the temperature of the image portion rises and the gain of the amplifying portion increases. FIG. 11 is a view of a white line detection portion control map provided in a determining portion of a white line detection apparatus according to related art. The white line detection portion control map uses the gain of the amplifying portion and the temperature of the imaging portion as variables, and includes an allowed region in which operation of the white line detection portion is allowed and a disallowed region in which operation of the white line detection portion is not allowed, based on the variables. The operation of the white line detection portion is controlled in accordance with the white line detection portion control map. In order to keep interference from thermal noise within an acceptable range, the white line detection portion control map according to the related art is set so that operation of the white line detection portion is not allowed at all if the temperature of the imaging portion becomes equal to or greater than a predetermined temperature, regardless of the gain of the amplifying portion, as shown in FIG. 11.
However, the temperature of the imaging portion increases during the daytime, when it is usually light outside and the gain of the amplifying portion is low. Therefore, it is difficult to imagine a case in which the temperature of the imaging portion and the gain of the amplifying portion are both high. Accordingly, during the daytime when the gain of the amplifying portion is low while the temperature is high, the white line detection apparatus according to the related art may stop operating without performing sufficiently even though performance-wise it is able to operate.
Also, Japanese Patent Application Publication No. 2005-175961 (JP-A-2005-175961) describes technology to prevent image quality from deteriorating when the temperature of an imaging device is high by limiting the range of image sensitivity (i.e., the gain when the image signal is amplified) if the temperature of the imaging device is high. However, limiting the gain in this way may make it more difficult to detect the white line when it is dark around the vehicle.